Die casting has long been known as a method of forming parts with complex geometries and/or surface ornamentation. Historically, the die casting of aluminum parts was commonplace in the automobile industry and many of the known methods have arisen from the needs of automobile manufacturers. Recently, the need to produce smaller, and more intricate, aluminum parts has arisen in the cell phone and electronics industries because such casings have excellent resistance to wear and work well to insulate internal components from the environment (heat, shocks, wetness, etc.). Aluminum parts also provide a smooth, metallic finish that allows for additional surface treatments, such as electroplating to enhance the quality and aesthetics of the parts. However, current methods of die casting aluminum parts do not adequately and consistently produce good results when being used to form smaller, more intricate parts.
Currently, the die casting of aluminum parts involves: pouring molten aluminum from a ladle into an injection shaft, plunging the molten aluminum through an external biscuit, up through a runner into the tool cavity. The tool cavity is located above the injection shaft in order to prevent the gravitational flow of molten aluminum into the tool cavity.
If the die casting machine is configured such that the injection shaft is located at the center of the tool cavity, some of the molten aluminum will flow though the force of gravity into the tool cavity prior to plunging the melt into the cavity. The resulting parts would have a poor surface finish and less dense microstructure due to the cooling of the molten aluminum which had leaked into the cavity prior to plunging the rest of the melt.
The aforementioned conventional method is shown in FIG. 3. The injection shaft is located beneath the tool cavity and the melt is plunged through a biscuit and travels upwards through a runner and then into the tool cavity. This casting method works well for larger parts, but results in a low yield when casting thin-walled parts. For such parts, as the melt travels upwards through the runner and into the tool cavity, it cools and loses both speed and pressure, thus causing flow marks and resulting in incomplete parts and parts with a poor microstructure and surface finish when forming smaller, more intricate parts. Many of these parts will either be scrapped and re-melted or will require secondary processing to make them acceptable.
U.S. Pat. No. 7,025,114, incorporated herein by reference in its entirety, also shows a similar method of die casting, but uses a three piece mold in order to obtain two-part mold structures. With reference to FIG. 3 of U.S. Pat. No. 7,025,114, a melt is poured into pouring port 343 which is then pressed upwards through a runner 33 by a plunger 341 before entering into cavity 32 via gates 312. Similarly to the aforementioned conventional method, the melt cools and loses both speed and pressure as it travels upwards through the runner and into the gates resulting in the same types of defects when casting smaller, more intricate parts. An additional problem with this method is that the melt will be cooler when it enters into the top gates than when it enters into the lower gates as it will have had to travel a greater distance, thus resulting in parts having a non-uniform density and poor microstructure. Therefore, there is a need for a method for die casting thin-walled parts with an open space within the part geometry that will result in a higher yield.